Amplifiers are used in the electronic industry for a variety of purposes. One common type of amplifier is a multi-stage amplifier in which a plurality of amplifier stages are connected together to provide desired levels of amplification. In one type of multi-stage amplifier, a field-effect transistor is provided in each stage. The field-effect transistor provides the amplifying effect of an input signal, and the amplified signal is provided as an input signal to the next stage. An example of such an amplifier is a monolithic microwave integrated circuit, which is particularly designed to amplify radio and microwave signals.
Such amplifiers often use a single bias pad connecting all gates or all drains of the field-effect transistors of each stage of the amplifier. The use of a single bias pad allows biasing of the gates or drains in each amplifier stage with a single applied voltage. In such amplifiers, both the gates and the drains are connected to ground through a bypass capacitor. The bypass capacitor is provided as a path for high frequency signals from ground to flow to ground in order to suppress transmission of the high frequency signals along the bias pad to adjacent amplifier stages. The bypass capacitor, however, blocks lower frequency signals from ground and allows lower frequency signals to be transmitted along the bias pad, such as a bias voltage. The bias pad is often implemented by connecting each stage's bypass capacitors together with microstrip lines to form a bias bus.
A problem with the use of such microstrip lines is that they form a resonant structure with the bypass capacitors. At resonance, this structure provides a low loss feedback path for signals into the gate or drain of the field-effect transistors, allowing crosstalk between the respective transistor terminals. This resonance creates a potential gain loop which often causes oscillation problems for the entire amplifier. The resonance also causes the impedance at the bypass capacitors to resonate from a short circuit to an open circuit (or large impedance). High impedances at low frequencies typically cause oscillations in microwave transistors. Prior methods for suppressing such effects of the gain loop are either space prohibitive or adversely affect the amplifier's output power and efficiency.